coils. The form of the armature being spherical, very little power is
lost by air friction, and no injury can occur from increased speed
developing centrifugal force. The field magnets, which surround the
armature, are cast iron shells, wound outside with many convolutions
of insulated copper wire, and are joined externally by iron bars to
convey the magnetism. These outer bars serve also as a most efficient
protection to the wire and armature of the machine during
transportation or otherwise. Objects cannot fall upon or rest upon the
wire coils and injure them. The coils of wire upon the field magnets
surround not only the iron poles or shells, but are situated also so
as to surround likewise the revolving armature, and increase the
effect produced in it by direct induction and magnetism. This feature
is not used in any other generator, nor does any other make use of a
spherical armature. The shaft is mounted in babbitted bearings of
ample size, sustained by a handsome frame therefor, and is of steel,
finely turned and perfectly true. The shaft and armature together are
balanced with the utmost care, and run without buzz or rumble. The
armature wire is kept cool by an active circulation of air over its
whole surface during revolution. The commutator, or portion from which
the currents developed in the armature are carried out for use, is a
beautiful piece of mechanism. It is mounted upon the end of the shaft,
and has attached to it the wires, three only, coming from the armature
wire through the tubular shaft.

[Illustration: THE THOMSON SPHERICAL.]

The commutator is peculiar, consisting of only three segments of a
copper ring, while in the simplest of other continuous current
generators several times that number exist, and frequently 120!
segments are to be found. These three segments are made so as to be